In general, heat pump devices to be used for a cooling and heating apparatus and a hot water supply device use air as a heat source.
Further, in regions where an outdoor air temperature is low, a heat pump device using underground heat in heating has also come to be used.
In an air heat-source heat pump device using heat of air as a heat source, when an outdoor air temperature is low in a heating operation, a heating capacity may be reduced due to reduction in suction pressure, frosting, or the like. Thus, operation efficiency of the heat pump device depends on the outdoor air temperature.
In an underground-heat heat pump device using underground heat, when an underground temperature is higher than an outdoor air temperature, the amount of collected heat can be increased, and hence the operation efficiency is higher than that of the air heat-source heat pump device. However, when the underground temperature is lower than the outdoor air temperature, conversely, the operation efficiency of the underground-heat heat pump device is worse than that of the air heat-source heat pump device.
Further, although the underground temperature varies less throughout the year as compared to the outdoor air temperature, the variation width of the underground temperature depends on the district, depth, and season. Also for this reason, the operation efficiency of the underground-heat heat pump device may be worse than that of the air heat-source heat pump device.
As a measure for solving those problems, in Patent Literature 1, there is disclosed a technology of switching between an air heat-source heat exchanger, which is installed on the ground to use outdoor air as a heat source, and an underground heat-source heat exchanger, which uses underground heat collected by an underground heat exchanger buried in the ground as a heat source. In Patent Literature 1, a passage is switched so that the air heat source exchanger may be used when an outdoor air temperature is equal to or higher than a predetermined value or when a refrigerant temperature is equal to or higher than a predetermined value (for example, equal to or higher than a temperature at which the air heat source exchanger is frosted) and that the underground heat-source heat exchanger may be used when the refrigerant temperature is equal to or lower than the predetermined value.